1. Technical Field
The present invention is directed to power supplies, and more particularly, to protection circuits for use with a controller.
2. Description of the Related Art
In power supply design, a critical feature for many end users is the ability of a power module to self-protect both itself and the circuitry it powers during fault or trouble conditions. There are several common trouble modes that must be accounted for in the design of a power module. These include output overload protection, short circuit protection, output overvoltage protection, and input undervoltage protection.
Output overload protection is required to protect the end user against excessive current and heating in either the power supply module or the end user""s circuitry due to either a misapplication or a damaged device. Output short circuit protection is required to protect the end user from excessive current and heating in either the power supply or the end user""s circuitry due to a failed device. Output overvoltage protection is required to protect the end user""s circuitry from being powered by excessively high voltage. This failure mode is due to either a failure in the power module""s control loop or a misapplication by the end user, e.g., adjusting an adjustable output voltage higher than the supply is rated for. Input undervoltage protection protects the end user against excessive input current and heating in either the power module or the end user""s circuitry due to the power module running at an input voltage that is lower than the supply is rated for.
It is a goal of the power supply designer to implement the required protection circuits in a manner that is both cost effective and space efficient. Various previous methods have attempted to address circuit protection in power supplies. In many designs, the power module continues to run using a different control mechanism, in others it enters a hiccup mode of operation, while in others the module shuts off until power is cycled. Regardless of the method of protection used, the protection circuits are implemented independently resulting in higher component count and cost, as well as higher space requirements.
A typical prior art protection circuit is shown in FIG. 4. This diagram depicts an undervoltage lockout circuit. The circuit consists of a first resistor 410 coupled to a second resistor 420 that is in turn coupled in parallel with a capacitor 430. The capacitor 430 is coupled to a first and second input of a universal voltage monitor (UVM) 440, which monitors undervoltage conditions. A third resistor 450 is coupled between the first resistor 410 and the output of the UVM 440. A fourth resistor 460 is coupled to a third input of the UVM 440 and a fifth resistor 470 is coupled between the fourth resistor 460 and the output of the UVM 440. Finally, the output of the UVM 440 is coupled to a power supply 480. An input voltage Vin is sensed and then compared to a reference voltage. The output of the UVM 440 will be zero when the input voltage is lower than a reference voltage. Once the output of the UVM 440 drops to zero, the power supply 480 will be disabled. Once the input voltage rises above the reference voltage, the power supply 480 will be reactivated. As can be seen, a significant number of components are used to implement this protection circuit.
It is desirable to merge protection features to achieve low component count and low cost. Some art exists which combines an undervoltage and over current protection circuit. Despite this combination, the component count is still high, though lower than if the circuits were independently implemented. This type of solution often latches the power module off and therefore requires power, cycling before restarting. For many users, the power cycling, requirement is undesirable. There is therefore needed in the art a new, lower component count and low cost power supply protection circuit.
The circuit demonstrating the present innovations is used to implement protection features with a minimal number of components. In a preferred embodiment, the innovative power module provides overcurrent (or overload), short circuit, output overvoltage, and input undervoltage protection. In a preferred embodiment, the protection circuit uses a hiccup mechanism where the module shuts off briefly and then attempts to auto-restart.
In a preferred embodiment, an operational amplifier compares a reference voltage to a voltage which is proportional to the, output voltage of the power supply. Under certain failure conditions, this produces an error signal which is compared to a second reference signal. Depending on the second comparison, a duty cycle exceeding a maximum or saturation of the control loop will cause the power supply to enter a hiccup mode wherein it repeatedly shuts itself down briefly then restarts to see if the. failure condition is gone.